Field of the Invention
This invention relates to distributed-scale apparatus for concentration and collection of solar energy.
Discussion of Prior Art
As non-renewable energy resources such as coal, oil, and natural gas become more scarce and costly to obtain, the need for practical ways to utilize renewable energy sources grows. Solar energy investment in industrialized regions has been influenced by preexisting patterns and infrastructure for distribution of electrical power from large, centralized facilities. Several large facilities for centralized solar thermal generation of electric power have been completed in the US and other countries. These have typically used a great number of ground-based heliostats to reflectively concentrate solar energy to a focal area containing a tower-mounted receiver. Thermal energy collected by the receiver is then is transferred to drive nearby steam turbines for generation of electric power, which is then fed into to the existing grid for distribution. Energy consumers in some areas have installed local systems, which avoid the transmission losses inherent in distribution from centralized plants, but usually operate at the lower efficiency of photovoltaic (PV) conversion. Locally collected solar thermal energy is also used for heating buildings or adjunctively to heat water. Such systems offer greater efficiency, avoiding both energy transmission and conversion losses. Local apparatus for concentrating solar thermal energy is much less common, though cooking is one relatively popular use.
Many less-developed areas of the world have seen significant deforestation and environmental degradation as a result of people using traditional fuels for cooking fires. Efforts have been made to encourage use of solar cookers in these areas, with limited success. However, there is an increasing need to encourage local use of solar energy in more industrialized regions, also. Populations with a lifestyle and economy that are more dependent upon fossil fuels will suffer greater stress, as those fuels become more costly. To the extent that means for renewable energy use are familiar and available to those populations, difficulties stemming from a culture of dependence upon fossil fuel can be mitigated.
Development and implementation of apparatus for local scale, distributed solar thermal energy collection and use confronts many of the same difficulties as large-scale installations. Apparatus for local concentration of solar energy offers a greater range of uses than non-concentrating solar collectors, but presents a greater challenge for design and cost efficiency. Beyond the costs of materials, manufacturing and maintenance, development of apparatus suitable for home, farm, or small business use also contends with issues such as space requirements, safety, portability and ease of use.
Prior art in this area has sometimes employed focusing lenses for concentration; an example is U.S. Pat. No. 4,913,130 (Inagaki, Sawata.) Reflecting concentrators have been more common, tending to be less costly. However, the incident angle of sunlight to a lens or reflector that is fixed in position will change continuously as the earth turns, thus changing the resulting reflective angle. A concentrator apparatus therefore must track the apparent movement of the sun across the sky, in order to achieve concentration of incident light to the desired target, or focal area, where the receiver of the apparatus is located.
Solar tracking by a concentrator must be done through two axes, corresponding to altitude and azimuth of the sun's apparent position in the sky. Maintenance of automated mechanical solar tracking has been a significant cost barrier for large, centralized solar thermal electric generation facilities. Concentrators with individually moving reflectors, even at a smaller scale, require a complicated mechanical infrastructure. An example is in U.S. Pat. No. 6,945,246 (Kinoshita).
However, smaller scale concentrators may also use reflective surface in the shape of an elliptic paraboloid, or a plurality of flat reflectors in a Fresnel array approximating the same effect. Such reflectors move upon a common framework to track the sun. This arrangement simplifies the mechanical infrastructure needed. “Parabolic dish” reflectors have been assembled from wedge-shaped sections of flat, polished sheet metal that are placed into a special curved frame to approximate a paraboloid shape, such as in U.S. Pat. No. 3,797,476 (Tarcici), and U.S. Pat. No. 6,863,065 (Marut, Brunette). An example of such parabolic dish concentration that has seen some commercial success is the German SK-series of cookers, designed by Dr. Dieter Seifert of EG Solar. At the time of this writing, information was accessible via: http://solarcooking.wikia.com/wiki/SK14. A similar design had begun production commercially in the US at the time of this writing, under the name “Sun Power Cooker” and information was accessible via: http://sunpowercooker.com/.
In dish type concentrators the receiver and the user can cast shadows over reflector area in some designs, reducing efficiency. Cooking vessels positioned over reflectors, or that must be lifted over them are more likely to may spill and spatter food upon the reflectors, also reducing efficiency. Some designs have partial paraboloid reflectors separated and foldable for storage or transport. An example of this is a parabolic cooker that has been subsidized and promoted by government in China, known as the Ao Chi F800. At the time of this writing, information was accessible via: http://solarcookingwikia.com/wiki/Ao_Chi_Solar_Cooker. The Ao Chi F800 reflective surface is not very durable, being a metallized plastic film. The film is produced in a flat sheet, and adhered to the curved, pre-cast paraboloid section after cutting the film into small pieces, to minimize wrinkles in the material. The receiver sits above the reflector area, with the resultant problems of spillage. This unit is also limited in its reflector size and power, in favor of user access to the receiver.
User access to the receiver for cooking-related activity becomes more problematic as the size of reflectors is increased for greater concentrator capacity. Similarly, the challenge of providing a convenient way to perform manual solar tracking adjustments grows in proportion to reflector area.
Fresnel reflector designs have been produced for the purpose of reducing reflector bulkiness and cost, such as in U.S. Pat. No. 4,350,412 (Steenblik) and U.S. Pat. No. 4,561,425 (Long, Ware). However, these implementations still require costly, laborious and difficult fabrication techniques, or lack a surrounding structure that provides good ease of use.
The prior art known to the applicant that is nearest in form to the current invention was based upon the “Papillon” Solar Cooker, developed by Jochen Dessel and Prof. Bernd Hafner of the Solarinstitut Jüich (Germany.) At the time of this writing, information on the Papillon was accessible at: http://solarcooking.wikia.com/wiki/Papillon and http://www.solar-papillon.com/. The Papillon has two reflector sections, similar to the Ao Chi design, but separated further from each other by a sizeable gap, with the receiver positioned above the gap. This avoids shadowing and food spillage problems. The Papillon's reflectors are of polished sheet aluminum in strips, fitted to a curved frame to approximate a continuous paraboloid section. Fabrication and assembly is costly, the paraboloid section shape is rather bulky, and the user approach to the focus becomes difficult when the sun is lower in the sky. The altitude adjustment mechanism of the Papillon comprises a sliding-groove and pinch-bolt device to attain and hold reflector position. The location of the mechanism is inconvenient and possibly hazardous to the user, as it requires reaching over the focal area past a hot cooking vessel. Azimuth adjustments require tilting the entire apparatus to pivot it upon its one axle or skidding its base foot sideways, risking spillage from the vessel at the focal area.
Lorin Symington of the Canadian non-profit corporation ASTRA modified the Papillon design, in part by using flat glass mirrors in a Fresnel-type array. This design was dubbed the “Iron Butterfly,” and demonstrated in West Africa in 2008. At the time of this writing, photo documentation was accessible via: http://solarcooking.wikia.com/wiki/Butterfly_(Iron).
The Iron Butterfly's reflector arrays are in a trapezoidal shape that does not maximize reflector aperture relative to outer dimensions of the apparatus, in use or in the (folded) storage position. Mounting and aligning the Fresnel array of mirrors requires a complex and heavy backing structure. A backing plate for each flat reflector in the array must be aligned and attached to framing with individual welded supports, making fabrication laborious and costly, and adding substantially to the weight of the apparatus. Information on the process for making the Iron Butterfly's reflector panels was accessible at the time of this writing via: http://www.astraonline.ca/?p=SFT. Though some success has been demonstrated, the difficult and laborious multi-step process is vulnerable to error and imprecision at various stages.
The Iron Butterfly's parallel-square reflector carriage features cross bracing removed to one end of the carriage, so that the user can closely approach and reach the focal area from the opposite end when the sun is at lower altitudes. But flexing and distortion of the non-braced portion of the carriage may reduce focal precision and usable energy, and the facilitation of user proximity to the focal area increases the likelihood of operator exposure to concentrated solar energy. The bracing location also limits the range of carriage travel; with the sun directly overhead, proper focus requires compensatory tilting of the entire apparatus. The Iron Butterfly's spooling mechanism for cable-controlled adjustment of reflector altitude is located near the focal area, similar to the Papillon design, so use is difficult and possibly hazardous. It requires the user to reach over the focal area and past a hot cooking vessel, or to walk around to the opposite side of the apparatus and duck or reach under reflector positioning lines, while stepping over the chassis/wheels. As in the case of the Papillon, azimuth and altitude adjustment also require the operator to gaze directly at the focal area to gauge the location and intensity of concentrated sunlight. The user position of the Papillon and Iron Butterfly is on the same side of the receiver that reflected, concentrated solar energy strikes when the sun is at lower altitudes, so that diffuse reflection to the user is greater. Protective eyewear may mitigate the discomfort and possible hazard of intense solar radiation reflected from the receiver to the user's eyes. However, positional adjustments are difficult and uncomfortable to perform accurately in this manner, further compromising ease of use and general efficacy of the concentrator.
Suspension lines to hold the Iron Butterfly's reflector panels in operating position are attached to the panel carriage asymmetrically and lack a means to maintain equal tension between them. This may contribute to flexion of the reflector panels with consequent focal imprecision.